Chronic viral infections such as HIV and HCV affecting millions of people globally are difficult to cure. Successful eradication of chronic viruses greatly depends on robust and long-lasting CD4 and CD8 T cell responses. Although virus-specific CD8 T cells gradually reduce their ability to produce effector cytokines and upregulate inhibitory receptor PD-1 in the face of persistent infection (commonly referred to as exhaustion), several studies, including the remarkable therapeutic effects achieved by PD-1 blockade, suggest that phenotypically ?exhausted? T cells can mediate a crucial level of pathogen control. In contrast to the presently accepted view of progressive deterioration, we propose here that virus-specific CD8 T cells stably adjust their lineage specification and undergo a functional adaptation, which is optimized to fulfill a certain level of effector function and pathogen control without causing overwhelming immunopathology. This new concept is further supported by our preliminary data. We have recently found that novel cellular and signaling pathways, which connect CD4-derived IL-21 to BATF-IRF4-STAT3 regulated transcriptional machinery in CD8 T cells, vigorously promote the early-to-late phase developmental transition in virus-specific CD8 T cells. Based on these new findings, we hypothesize that elevated inflammatory cytokines (such as IL-21) and persistent antigen as hallmarks of chronic viral infection are tightly coupled through IL-21-STAT3-BATF and Antigen- TCR-IRF4 pathways, which converge on a transcriptional level to collaboratively regulate gene expression. Supporting this idea, our computational analyses have revealed that BATF and IRF4 cooperatively bind to the cis-regulatory elements of multiple genes associated with the late phase effector signatures. Intriguingly, many of these BATF-IRF4 compound-binding elements are adjacent to STAT3 binding sites, which provide the structural basis of BATF-dependent STAT3 binding and transcriptional activation as evidenced in our preliminary studies. Together, these lead us to propose a provocative model, in which BATF and IRF4 form a central regulatory hub to modulate chromatin accessibility and cooperate with STAT3 to regulate the central transcriptional networks that govern the late phase effector CD8 T cell differentiation and function. In this grant, we will use state-of-the-art techniques to elucidate (1) which subset of CD4 T cells provides ?help? to sustain the effector function in CD8 T cells, (2) whether temporally regulated BATF expression and antigen dependent IRF4 expression are cooperatively required for CD8 effector T cell differentiation at the late phase of chronic infection, and (3) how BATF-IRF4 complex cooperates with JAK-STATs pathways on the epigenetic level to confer a distinct cell-fate specification in late effector CD8 T cells. Overall, this work will provide mechanistic insights into how CD8 T cells integrate the cellular, molecular and genetic signals to stably adjust their differentiation process to meet the needs of chronic infection. Ultimately, knowledge gained from this study could be harnessed to improve the effector function of CD8 T cells in treating chronic viruses and cancer.